Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula HO−C(=O)−C(=O)−OH, also written as (COOH)2 or (CO2H)2 or H2C2O4. It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name is derived from early investigators who isolated oxalic acid from flowering plants of the genus Oxalis, commonly known as wood-sorrels. It occurs naturally in many foods. It can be toxic when ingested in significant quantities, and contact with concentrated forms can cause chemical burns. Oxalic acid is a much stronger acid than acetic acid. It is a reducing agent and its conjugate bases hydrogenoxalate (HC2O−4) and oxalate (C2O2−4) are chelating agents for metal cations. It is used as a cleaning agent, especially for the removal of rust, because it forms a water-soluble ferric iron complex, the ferrioxalate ion. Oxalic acid typically occurs as the dihydrate with the formula H2C2O4·2H2O.
History The preparation of salts of oxalic acid from plants had been known since at least 1745, when the Dutch botanist and physician Herman Boerhaave isolated a salt from wood sorrel, akin to kraft process.
By 1773, François Pierre Savary of Fribourg, Switzerland had isolated oxalic acid from its salt in sorrel. In 1776, Swedish chemists Carl Wilhelm Scheele and Torbern Olof Bergman produced oxalic acid by reacting sugar with concentrated nitric acid; Scheele called the acid that resulted socker-syra or såcker-syra (sugar acid). By 1784, Scheele had shown that "sugar acid" and oxalic acid from natural sources were identical. The modern name was introduced (along with many other acid names) in 1787, by de Morveau, Lavoisier and co-authors. In 1824, the German chemist Friedrich Wöhler obtained oxalic acid by reacting cyanogen with ammonia in aqueous solution. This experiment may represent the first synthesis of a natural product.
Production
Industrial Oxalic acid is mainly manufactured by the oxidation of carbohydrates like glucose using nitric acid or air in the presence of vanadium pentoxide. Another process uses oxygen to regenerate the nitric acid, using a variety of precursors including glycolic acid and ethylene glycol. As of 2011, this process was only used by Mitsubishi in Japan. A newer method entails oxidative carbonylation of alcohols to give the diesters of oxalic acid:
4 ROH + 4 CO + O2 → 2 (CO2R)2 + 2 H2O These diesters are subsequently hydrolyzed to oxalic acid. Approximately 120,000 tonnes are produced annually. Historically oxalic acid was obtained exclusively by using caustics, such as sodium or potassium hydroxide, on sawdust, followed by acidification of the oxalate by mineral acids, such as sulfuric acid. Oxalic acid can also be formed by the heating of sodium formate in the presence of an alkaline catalyst.
Laboratory Although it can be readily purchased, oxalic acid can be prepared in the laboratory by oxidizing sucrose using nitric acid in the presence of a small amount of vanadium pentoxide as a catalyst. The dihydrate can be converted to the anhydrous form by heating or azeotropic distillation.
Structure
Anhydrous Anhydrous oxalic acid exists as two polymorphs; in one the hydrogen-bonding results in a chain-like structure, whereas the hydrogen bonding pattern in the other form defines a sheet-like structure. Because the anhydrous material is both acidic and hydrophilic (water seeking), it is used in esterifications.
Dihydrate The dihydrate H2C2O4·2H2O has space group C52h–P21/n, with lattice parameters a = 611.9 pm, b = 360.7 pm, c = 1205.7 pm, β = 106°19′, Z = 2. The main inter-atomic distances are: C−C 153 pm, C−O1 129 pm, C−O2 119 pm.
Reactions
Acid–base properties Oxalic acid's pKa values vary in the literature from 1.25 to 1.46 and from 3.81 to 4.40. The 100th ed of the CRC, released in 2019, has values of 1.25 and 3.81. Oxalic acid is relatively strong compared to other carboxylic acids:
Oxalic acid undergoes many of the reactions characteristic for other carboxylic acids. It forms esters such as dimethyl oxalate (m.p. 52.5 to 53.5 °C, 126.5 to 128.3 °F). It forms an acid chloride called oxalyl chloride.
Metal-binding properties Transition metal oxalate complexes are numerous, e.g. the drug oxaliplatin. Oxalic acid has been shown to reduce manganese dioxide (MnO2) in manganese ores to allow the leaching of the metal by sulfuric acid. Oxalic acid is an important reagent in lanthanide chemistry. Hydrated lanthanide oxalates form readily in very strongly acidic solutions as a densely crystalline, easily filtered form, largely free of contamination by nonlanthanide elements:
2 Ln3+ + 3 H2C2O4 → Ln2(C2O4)3 + 6 H+ Thermal decomposition of these oxalates gives the oxides, which is the most commonly marketed form of these elements.
Other Oxalic acid and oxalates can be oxidized by permanganate ion in an autocatalytic reaction. Oxalic acid vapor decomposes at 125–175 °C (257–347 °F) to CO2 and formic acid (HCOOH). Photolysis with 237–313 nm UV light also produces carbon monoxide (CO) and water. Evaporation of a solution of urea and oxalic acid in 2:1 molar ratio yields a solid crystalline compound H2C2O4·2CO(NH2)2, consisting of stacked two-dimensional networks of the neutral molecules held together by hydrogen bonds with the oxygen atoms.
Occurrence
Biosynthesis At least two pathways exist for the enzyme-mediated formation of oxalate. In one pathway, oxaloacetate, a component of the Krebs citric acid cycle, is hydrolyzed to oxalate and acetic acid by the enzyme oxaloacetase:
[O2CC(O)CH2CO2]2− + H2O → C2O2−4 + CH3CO−2 + H+ It also arises from the dehydrogenation of glycolic acid, which is produced by the metabolism of ethylene glycol.
Occurrence in foods and plants
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